Objectives Assess whether relationships of juvenile marmosets with members of the natal family determine future social styles and the ability to attain the dominant, breeding position in a social group. We have previously found that adult female marmosets show pronounced individual differences in their aggressiveness and submissiveness toward unfamiliar females and in their likelihood of attaining dominance in a newly formed social group. In this study we are evaluating whether these individual differences in social style are determined by differences in early social experience of females, particularly relationships with members of the natal family. One hypothesis is that females which establish themselves as dominant over their co-twin will exhibit generalized aggressiveness toward unfamiliar conspecifics and therefore will be more likely to attain a dominant, breeding position in a social group, whereas females which are subordinate to the co-twin will be more likely to adopt a submissive demeanor towards conspecifics. Thus, this hypothesis predicts that female co-twins will exhibit contrasting behavioral profiles in tests of social responsiveness. An alternative hypothesis is that overall family style is particularly influential in determining social styles of young females. According to this hypothesis, co-twins should exhibit similar response styles toward strangers. Subjects are 28 marmosets, comprising 7 male-female and 7 female-female twin-pairs housed with their natal families. Focal-animal behavioral observations of 15 min are conducted on each subject four times per week from ages 2-8 months, twice per week from 8-12 months, and twice per month from 12-18 months. Behaviors scored include agonistic interactions, affiliative interactions, play, and food-related interactions. Of particular interest are the degree of social initiative or passivity exhibited by subjects and agonistic interactions between twins. At the conclusion of this observational study, subjects undergo a series of stranger-encounter tests and twin-dominance tests, prior to being introduced into peer groups of unrelated animals. Key words ontogeny, social development, social relationships, dominance, temperament